Why do we have leap years?

A leap year has an extra day compared to a regular year, February 29th (Illustrative image: Pinterest).

Normally, a Gregorian year has 365 days, but a leap year has 366 days. A leap year occurs every four years. Other calendars, such as the lunar calendar, the Jewish calendar, and the Islamic calendar, also have leap years, but not according to the four-year cycle. Some calendars also have leap days or leap months.

Besides leap years and leap days, the Gregorian calendar also has leap seconds, meaning that occasionally a few seconds are added to certain years. The most recent leap seconds were added in 2012, 2015, and 2016. However, the International Bureau of Weights and Measures (IBWM), responsible for global timekeeping, will abolish leap seconds from 2035 onwards.

Why do we need leap years?

On the surface, adding an extra year to a year might seem like a silly idea, but leap years are actually very important.

We have leap years because a normal solar year is slightly shorter than a solar year – that is, the time it takes for the Earth to complete one orbit around the Sun. A normal solar year is exactly 365 days long, but a solar year is approximately 365.24 days, or in other words, 365 days, 5 hours, 48 ​​minutes, and 56 seconds.

If we disregard this difference, each year the gap between the start of a solar year and the beginning of a solar year increases by 5 hours, 48 ​​minutes, and 56 seconds. For example, if we omit leap years, after about 700 years, summer in the Northern Hemisphere would begin in December instead of June.

The Earth takes 365.24 days to orbit the Sun, slightly longer than a typical solar year. (Image: Getty Images)

Thanks to the extra day in a leap year, that problem is solved. However, this adjustment system isn't entirely accurate. Every four years, we gain approximately 44 minutes, equivalent to one extra day every 129 years.

To address this issue, every hundred years we omit a leap year, except for years divisible by 400 such as 1600 and 2000. But even so, there is still a small difference between the Gregorian year and the solar year, so IBWM has tried adding leap seconds.

When did we start having leap years?

The idea of ​​incorporating leap years into the calendar dates back to 45 BC, when the ancient Roman Emperor Julius Caesar promulgated the Julian calendar, and Rome used it from 46 BC onwards. According to the Julian calendar, each year had 445 days divided into 15 months, and there were leap years every four years, synchronized with the seasons on Earth.

For centuries, the Julian calendar was used routinely, but by the mid-16th century, astronomers noticed that the seasons began about 10 days earlier than important holidays such as Easter, and were no longer compatible with transitional periods like spring or the vernal equinox.

To correct this, Pope Gregory XIII promulgated the Gregorian calendar, which is the solar calendar we use today, in 1582. Essentially, the solar calendar is similar to the Julian calendar but omits the leap year every 100 years, as explained above.

For centuries, only Catholic countries like Italy and Spain used the Gregorian calendar, but eventually Protestant countries like England also switched to it in 1752.

Due to significant differences between various calendars, when these countries switched to the Gregorian calendar, they had to omit many days of their own year to synchronize with other countries. For example, when England changed its calendar in 1752, September 2nd was followed by September 14th, skipping the days from September 3rd to 13th.

At some point in the distant future, the Gregorian calendar may be re-evaluated as it will no longer be synchronized with the solar year, but that will likely not happen for thousands of years.

According to LiveScience

Source: https://dantri.com.vn/khoa-hoc-cong-nghe/do-dau-chung-ta-co-nam-nhuan-20240209030429198.htm